A thread clamp of this nature is usually mounted on one side of the loom frame as close as possible to an insertion nozzle in which a cut-off end of the weft yarn remains while the shed is being reformed, this positioning being desirable in order to prevent an accidental withdrawal of the weft end from the nozzle due to the elasticity of the yarn. Reference in this connection may be made, for example, to commonly owned application Ser. No. 331,922 filed Dec. 17, 1981 by one of us --Albert Henri Deborde--jointly with another, now U.S. Pat. No. 4,448,223.
Conventionally, thread clamps used for the purpose described include a fixed and a movable jaw, the latter being operable by a single-acting pneumatic jack to which air under pressure is applied when the clamp is to close around the weft yarn; when the air pressure is relieved, a countervailing biasing spring reopens the clamp. Such an arrangement has the drawback that the exerted clamping pressure is a function of the air pressure which may be subject to significant variation, e.g. when the loom includes two or more nozzles for the alternate insertion of different weft yarns. Moreover, the reopening of the clamp by means of a biasing spring is somewhat sluggish which, since the release of the weft must be precisely synchronized with the loom cycle, impedes high-speed operation.